Static hazard detector for isolated alternating current supply lines

ABSTRACT

First and second resistors are connected in series with respective capacitors at first and second junction points. The two R-C series combinations are connected between the two line conductors of an isolated AC supply line, with each capacitor being connected to a different line conductor. Each junction point is connected to one terminal of a respective full wave rectifier, and the opposite terminals of the two full wave rectifiers are connected to a third junction point. Current flow through each rectifier is controlled by a respective transistor, and the transistors are alternately triggered conductive by an oscillator. A photoelectric resistor, such as a cadmium sulphide resistor, is connected in series between the third junction point and one terminal of a third full wave rectifier having an AC potential supply thereto so as to serve as an interposed source of DC potential. A lamp subjected to the voltage of the supply line is operatively associated with the photoelectric resistor so that the impedance of the detector circuit, or the &#39;&#39;&#39;&#39;source impedance,&#39;&#39;&#39;&#39; is varied so that the detector current is maintained at a constant value over the entire voltage range of from 102v to 132v, for a nominal 120v AC supply. The other terminal of the DC source is connected to one terminal of a milliammeter whose opposite terminal is grounded, and diode means are connected to the millianmeter so that fault current can flow therethrough in only one direction. The milliammeter, through the mentioned circuitry, is connected alternately to the first and second junction points for flow of fault current alternately from the line conductors in only one direction to ground through the milliammeter. A switching relay controls signal lights and an alarm, and normally maintains a &#39;&#39;&#39;&#39;safe&#39;&#39;&#39;&#39; lamp illuminated. Upon occurrence of a fault current flow through the milliammeter, an amplifier amplifies the fault current to energize the relay to switch connections to sound an alarm, to illuminate a &#39;&#39;&#39;&#39;hazard&#39;&#39;&#39;&#39; lamp, or to do both. Phase shift adjustment means are associated with the R-C series combinations, and a diode is connected in by-pass relation with the third full wave rectifier to direct reverse current flow in by-pass relation to the third full wave rectifier.

Uiteti State-s atent Legatti [72] Inventor:

[54] STATIC HAZARD DETECTOR FOR ISOLATED ALTERNATING CURRENT SUPPLYLINES Raymond H. Legatti, Clearwater, Fla.

Electromagnetic Industries, Inc., Clear- 1 water, Fla.

22 Filed: May 13, 1971' [21] Appl.No.: 143,096

[73] Assignee:

Souillard Warrington ..317/27 R Primary Examiner-James D TrammellAttorney-John J McGlew and Alfred E. Page [57] ABSTRACT First and secondresistors are connected in series with respective capacitorsat first andsecond junction points. The two R- C series combinations are connectedbetween the two line conductors of an isolated AC supply line, with eachcapacitor [451 May 30, 1972 being connected to a different lineconductor. Each junction point is connected to one terminal of arespective full wave rectifier, and the opposite terminals of the twofull wave rectifiers are connected to a third junction point. Currentflow through each rectifier is controlled by a respective transistor,and the transistors are alternately triggered conductive by anoscillator. A photoelectric resistor, such as a cadmium sulphideresistor, is connected in series between the third junction point andone terminal of a third full wave rectifier having an AC potentialsupply thereto so as to serve as an interposed source of DC potential. Alamp subjected to the voltage of the supply line is operativelyassociated with the photoelectric resistor so that the impedance of thedetector circuit, or the source impedance, is varied so that thedetector current is maintained at a constant value over the entirevoltage range of from 102v to 132v, for a nominal 120v AC supply.

The other terminal of the DC source is connected to one terminal of amilliammeter whose opposite terminal is grounded, and diode means areconnected to the millianmeter so that fault current can flowtherethrough in only one direction. The milliammeter, through thementioned circuitry, is connected alternately to the first and secondjunction points for flow of fault current alternately from the lineconductors in only one direction to ground through the milliammeter. Aswitching relay controls signal lights and an alarm, and normallymaintains a safe lamp illuminated. Upon occurrence of a fault currentflow through the milliammeter, an amplifier amplifies the fault currentto energize the relay to switch connections to sound an alarm, toilluminate a hazard" lamp, or to do both. Phase shift adjustment meansare associated with the RC series combinations, and a diode is connectedin by-pass relation with the third full wave rectifier to direct reversecurrent flow in by-pass relation to the third full wave rectifier.

12 Claims, 2 Drawing Figures Patented May 30, 1972 2 Sheets-Sheet 1INVENTOR. fidrm/vfl M [5 ,4727

Patented May 30, 1972 2 Sheets-Sheet 2 INVENTOR.

firmly/ya A4 Asanrr/ STATIC HAZARD DETECTOR FOR ISOLATED ALTERNATINGCURRENT SUPPLY LINES BACKGROUND OF THE INVENTION As mentioned in Kusterset al., US. Pat. No. 2,999,231 and McKinley'et al., US. Pat. No.3,066,284, both assigned to the National Research Council of Canada, theNational Fire Protection Association recommends use of an ungroundedalternating current electrical distribution system in electricallysensitive areas such as, for example, hospital operating rooms whereanaesthetic is administered, delivery rooms, coronary care units,intensive care units, cardiac catheterization laboratories, angiographiclaboratories, recovery rooms, emergency rooms, and dialysis units, andhas recommended also that a Line Isolation Monitor" be so arranged thata green signal lamp is displayed conspicuously so as to be visible topersons in the anaesthetizing locations, with such lamp to remain litwhile the system remains isolated safely from ground. .If a groundingimpedance or fault of less than a predetermined value should becomeconnected between any line conductor and ground, as for example by aconnection having a resistance from to 120,000 ohms, a red signal lampand an audible warning signal is required to be energized. The maximumcurrent that may flow in a low resistance fault by virtue of thepresence of the detector is specified not to exceed 2 milliamperes. e I

The two mentioned patents disclose hazard detectors meeting thesespecifications, and also discuss the various types of fault impedanceswhich maybe encountered, such as purely resistive fault impedances,purely capacitive fault impedances, and various combinations ofresistive and capacitive fault impedances. The magnitude of the maximumground current which can be produced by deliberately choosing the worselocation for a fault is a measure of the hazard of the system, and istermed, in the mentioned patents, the Hazard Index" which can bequantitatively expressed in milliamperes. As also mentioned, anypractical ground detector will obviously introduce a finite impedancebetween the system and ground, and the hazard index of an idealungrounded system with a ground detector associated therewith will nolonger be zero. The actual index of such an ideal ungrounded systemincluding a ground detector is called the Detector Hazard Index," andthe Total Hazard Index" of a practical ungrounded supply with a grounddetector connected therein is the sum of the AC currents due to faultimpedances and the detector impedance, with the addition being effectedvecton'ally.

Practical hazard detectors embodying the disclosures of the twoU.S.,patents mentioned above are available commercially, and meet thestandards set forth by the National Fire'Protection Association.However, the rate of switching between the two line conductors is of theorder of only 2cps, and the Detector Hazard Index" is of the. order ofone-half of the Total Hazard Index. In some cases, this relatively lowswitching rate' can be uncomfortable to a patient on the operatingtable.

SUMMARY OF THE INVENTION This invention relates to fault detectorcircuits for use with AC supply lines having line conductors isolatedfrom ground and, more particularly, to an improved, simplified, moresensitive and more compact fault detector circuit or hazard detectorcircuit.

In accordance with the invention, a first capacitor and a first resistorare connected in series at a first junction point, to provide a firstR-C combination, and a second capacitor and a second resistor areconnected in series at a second junction pointQto provide a second R-Ccombination. The two R-C combinations are connected between the lineconductors of the AC supply line, with one capacitor being connecteddirectly to one line and the'other capacitor being connected directly tothe other line. Each resistor has an impedance substantially equal inmagnitude to that of one of the capacitors at the supply line frequency.Respective electronic switching and rectifying means connect the firstand second junction points to a third common junction points, and areoperable toeffect current flow from the first and second junction pointsalternately and cyclically to the third junction point.

A variable impedance is connected in series between the third junctionpoint and a source of DC potential, such as a full wave rectifier havingan AC potential applied thereto from the supply line, and this impedancehas an impedance magnitude which varies directly with the supply linevoltage. In effeet, a boosting DC potential is injected into the circuitincluding the impedance connected to the third junction point. Amilliammeter is connected in series between the other terminal of theinjected or interposed DC source and ground, for alternating connectionto the first and second junction points for flow of fault currentalternately from the line conductors in only one direction to groundthrough the milliammeter. Diode means are connected to the milliammeterand block current flow through the milliammeter in the reversedirection.

A visible and audible alarm and warning system is connectedsubstantially in parallel with the milliammeter, and includes anamplifier for the current through the'milliammeter and a relay which iscontrolled by the amplifier. This relay has contacts controllingenergization of a safe" lamp, a hazard" lamp and a buzzer.

The detector circuit has a maximum of SOO-microamperes detector hazardindex for a total hazard index of 2 milliamperes, which is one-quarteror less of the total index. The electronic switching and rectifyingmeans effect switching between the two line conductors at 37 cps, whichis more beneficial to a patient on an operating table than is theswitching at 2 cps or less, which can be very annoying to such apatient. In addition, the variable impedance connected between the thirdjunction point and the full wave rectifier maintains the detectorcurrent at a constant value over the entire voltage range for a nominalv supply line voltage,

within the limits of a 15 percent decrease in voltage, or 102v, or a 10percent increase in voltage, or 132v.

An object of the invention is to provide an improved fault detectorcircuit for ungrounded AC supply lines.

Another object of the invention is to provide such a fault detectorcircuit which is simpler, more compact, and has a lower detector hazardindex than known fault detector circuits.

A further object is to provide such a fault detector circuit in whichthe detector current is maintained at a constant value over the entirevoltage range of the AC supply line.

Another object of the invention is to provide such afault detectorcircuit having a switching frequency which is much higher than thosehitherto used and is not annoying to a pa-- tient on an operating table.I

A further object of the invention is to provide such a fault detectorcircuit which is simple and compact in construction and relativelyinexpensive.

For an understanding of the principles of the invention, reference ismade to the following description of typical embodiments thereof asillustrated in the accompanying drawing.

BRIEF DESCRIPTIONOF THE DRAWING In the drawing:

FIG. I is a schematic wiring diagram of one form of fault detectorcircuit embodying the invention; and

FIG. 2 is a schematic wiring diagram of a preferred form of faultdetector circuit embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, anAC supply line, having line conductors L1 and L2 isolated from ground,has an AC potential of, for example, 120v, applied thereto at terminals10 and 11. The AC supply line is subject to probable connection toground by any combination of resistive and capacitive fault impedances.The fault detector circuit embodying the invention is connected betweenisolated line conductors L1 and L2 and ground.

In accordance with the invention, a first resistor R1 is connected inseries with the first capacitor C1 at a junction point JPl, withresistor R1 being connected to line L1 and capacitor C1 being connectedto line L2. Also, a resistor R2 is connected in series with a capacitorC2 at a junction point JP2, with resistor R2 being connected to line L2through resistor R3, and capacitor C2 being connected to line Ll. Aconductor 12 connects junction point JPl to one terminal of a rectifierbridge BRl through surge limit resistor RSLl, and a conductor 13connects junction point J P2 to one terminal of a rectifier bridge BR2through surge limit resistor RSLZ. The opposite terminals of therectifier bridges BR1 and BR2 are connected by respective conductors 14and 15 to a common junction point JP3. Flow of current from conductor 12to conductor 14 through rectifier bridge BRI is controlled by atransistor Ql whose collector-emitter circuit is connected across theother two terminals of rectifier bridge BR]. Similarly, flow of currentfrom conductor 13 to conductor 15, through rectifier bridge BR2, iscontrolled by a transistor Q2 whose collectoremitter circuit isconnected across the other two terminals of rectifier bridge BR2. RSL2and RSL3 are two additional surge current limiting resistors whichsuppress the transients produced by the fast semi-conductor switching.

A transformer T1 has its primary winding P1 connected across isolatedline conductors L1 and L2. Transformer T1 has secondary windings S1 andS2. For a purpose to be described, secondary winding S1 is connectedacross a pair of opposite terminals of a rectifier bridge BR3. Secondarywinding S2 has a rectifier bridge BRS connected between its opposite endterminals.

A second transformer T2 has primary windings P3 and P4 and secondarywindings S3 and S4, polarized as indicated by the dots. A conductor 16connects one DC output terminal of bridge BRS to the midpoint of primarywinding P3 and,

through a resistor R21, to the midpoint of winding P4. A conductor 17connects the positive DC output terminal of bridge BRS, through seriesresistors R13 and R16, to a conductor 18 connected in common to theemitters of a pair of transistors Q3 and Q4 constituting an oscillator.The collectors of transistors Q3 and Q4 are connected to respectiveopposite terminals of primary winding P3, and their bases are connectedto respective opposite terminals of primary winding P4. Secondarywinding S3 of transformer T2 is connected across the base-emittercircuit of transistor Q1, and secondary winding S4 is connected acrossthe base-emitter circuit of transistor Q2.

With the arrangement thus described, transistors Q1 and Q2 are triggeredconductive alternately and cyclically to correspondingly renderrectifier bridges BRI and BR2 conductive alternately and cyclically sothat current will flow from junction points .lPl and JP2 to junctionpoint JP3 alternately and cyclically. Thus, the isolated line conductorsL1 and L2 are sampled," for fault impedances, alternately andcyclically. Preferably, the oscillator comprising transistors Q3 and Q4,and their associated components, effects switching between isolated lineconductors L1 and L2 alternately and cyclically at 37 cps. A capacitorC9, connected between conductors l6 and 17, filters the DC outputvoltage of rectifier bridge BRS, as to remove ripples therefrom. A Zenerdiode Z8, connected between conductors 16 and 17 through resistor R13maintains a voltage to the oscillator and amplifier, which issubstantially constant. A diode D9 is connected between conductors l6and 18 for improved regulation of the oscillator voltage.

The two terminals of bridge BR3, connected to the opposite ends ofsecondary winding S1 of transformer T1, will be referred to as the ACinput terminals. Secondary winding S1, in conjunction with rectifierbridge 8R3, serves to inject or interpose an additional DC voltage inseries with the voltage at junction point JP3. For this purpose, animpedance, comprising a resistor R5 connected in parallel with acapacitor C6, is connected between junction point JP3 and a junctionpoint 1P4 which is, in turn, connected through a series resistor R7 to aDC input terminal of rectifier bridge BR3. For a purpose to bedescribed, a diode D2, polarized reversely to rectifier bridge BR3, isconnected between the DC output terminal of rectifier bridge BR3 andjunction point 1P4. Preferably, the impedance comprising the resistor R5and the capacitor C6 is a voltage variable impedance, whose impedancevalue increases with increasing voltage between isolated line conductorsL1 and L2, so as to maintain the detector current substantially constantover the full range of voltages which, based upon a nominal v AC supply,with a 15 percent decrease and a 10 percent increase, ranges from l02vto l32v AC.

The DC output terminal of rectifier bridge BR3 is connected by aconductor 19 to an input terminal of rectifier bridge 8R4, and theopposite output terminal of rectifier bridge BR4 is connected to groundby a conductor 20. A capacitor C2 is connected between the DC inputterminal of rectifier bridge 8R3 and the grounded output terminal ofrectifier bridge BR4, for phase correction. The fault impedancedetection circuit is connected across the other two terminals ofrectifier bridge 8R4.

This fault detecting circuit includes an adjustable resistor R8connected between one of these two terminals of bridge BR 4 and aterminal of an indicating instrument, such as a mil? liammeter MA, andthe other terminal of milliammeter MA, which is the negative terminal,is connected to the base of a transistor Q5. A filter capacitor C7 isconnected between the positive terminal of milliammeter MA and theemitter of NPN transistor Q5, at a junction point JPS. A resistor R17connects junction point JP5 to a conductor 20 connected to conductor 16at its junction point JP6 with capacitor C9. Conductor 20 is connected,through a Zener diode 29 in parallel with a resistance R6,to thenegative output terminal of bridge BR4.

By virtue of the interposition of a DC potential between the junctionpoint JP3 and meter MA, by the bridge 8R3, there is always a smallcurrent flow through meter MA even in the absence of a fault. In theevent of a balanced fault, the interposed source of DC potential resultsin the resistances on one side being effectively connected in seriesrather than in parallel. One of these resistances would be the resistorR1 or the resistor R2, and the other resistance would be the faultimpedance, acting as a resistive fault. Consequently, the detectorcircuit is able to detect balanced faults, either resistive faults orcapacitive faults.

' The secondary winding S2 of transformer T1, in association withrectifier bridge BR5, in addition to supplying the operating potentialfor the oscillator comprising transistors Q3 and Q4, also suppliespotential for a relay operating coil Kl which transfers, upon detectionof a fault, to extinguish a "safe" lamp, to light a hazard" lamp and toenergize a warning buzzer. Only the relay operatingcoil K1 is shown inFIG. 1, and the circuits controlled thereby will be described further inconnection with FIG. 2 which illustrates these circuits. 7

The signalling circuit comprises a conductor 21 connected to thecollector of transistor Q5 and, through a resistor R15, to the base of atransistor Q6 forming part of an amplifier for amplifying the relayoperating current. Thus, when transistor Q5 is triggered conductive bythe flow of a fault current through meter MA, transistor Q6 will also betriggered conductive. A zener diode Z6 and a resistor R10 are connectedbetween junction point JPS and a conductor 22 connected betweenconductor 17 and one terminal of relay operating coil K1. Zener diodeZ6, in associated with resistors R10 and R17, constitutes a compensatingcircuit, with Zener diode 26 maintaining a constant voltage and bridgeBRS supplying a variable voltage. The voltage drop provided by theresistors R10 and R17 is linear, as the voltage across Zener diode Z6 isconstant. Resistors R10 and R17 are selected for the right proportion ofthe correction. The voltage level of Zener diode 26 sets the slope ofthe correction.

Resistor R16 in series with diode D9 connects the emitter of transistorQ6 to the common conductor 16, and a Zener diode Z8 is connected inparallel with resistor R16 and diode D9 to maintain a substantiallyconstant voltage between the collector of transistor Q6 and conductor16. A capacitor C8 is connected between the emitter and base oftransistor Q6. The collector of transistor Q6 is connected, in serieswith a resistor R18, to the base of a transistor Q7 whose collector isconnected to the other terminal of relay operating coil K1. The emitterof transistor O7 is connected to the common conductor 16, and a resistorR19 in parallel with a capacitor C is connected between the base andemitter of transistor Q7. A smoothing capacitor C5 is connected acrossrelay operating coil Kl along with back EMF diode D5.

Transistors Q6 and Q7, with their associated components, constitute anamplifier having its operating potential supplied from secondary windingS2 through bridge BRS. The contacts controlled by relay operating coilK1 are normally in a position where a safe" lamp is illuminated, ahazard lamp is not illuminated and a warning buzzer is silent. Whenmilliammeter MA registers a hazardous fault current, transistor O5 istriggered conductive to, in turn, trigger transistor Q6 conductive whichalso triggers transistor Q7 conductive so that an energizing currentwill flow through relay operating coil K1 and the collector-emittercircuit of transistor Q7 to transfer the contacts controlled by relayoperating coil K1. This extinguishes the safe" lamp, illuminates thehazard lamp and energizes the warning buzzer.

A preferred embodiment of the invention is shown in FIG. 2. This figurealso illustrates the switches, lamps and buzzer controlled by relayoperating coil K1. The embodiment of H62 differs from that of FIG. 1 inthat the impedance, comprising the capacitor C6 in parallel with theresistor R5, con.- nected between junction point .IP3 and junction pointJP4 is replaced by a supply voltage responsive impedance comprising aphoto-resistor PR in parallel with an adjustable resistor R5.Photo-resistor PR may be, for example, a cadmium sulphide clement, whichhas light directed thereupon from a photo-resistor lamp PRL whoseillumination is a function of the supply voltage. The arrangement issuch that, as the supply voltage increases, the illumination of lamp PRLdecreases so that the resistance of photo-resistor PR increases, andviceversa. With a nominal supply voltage of 120v AC, the sourceimpedance is varied so that the detector current is maintained at aconstant value over the entire voltage range from l02v to l32v,representing a l5 percent decrease in the nominal voltage and a 10percent increase therein. As is well understood by those skilled in theart, a cadmium sulphide photo-resistor is a pure resistance.

In view of the rather detailed description of FIG. 1, it is believedthat only a description of the differences of FIG. 2, with respect toFIG. 1, need be mentioned. While there are minor changes in certaincomponents, such as resistors and capacitors, in FIG. 2, as compared toFIG. 1, these do not in any way effect the underlying principle ofoperation of the hazard detector of the invention.

Referring to FIG. 2, a Zener diode ZD is connected to conductor 16,which conductor is connected to the midpoints of windings P3 and P4 oftransformer T2, and, through a resistor R30, to the base of a transistorOS. A resistor R31 connects the emitter of transistor Q8 to the commonconductor 16. The collector of transistor Q8 is connected to oneterminal of photo-resistor lamp PRL, whose other terminal is connectedto a conductor 23 connected to an output terminal of rectifier bridgeBRS. A resistor R32 is connected in parallel with lamp PRL and thecollector emitter circuit of transistor Q8.

With the described arrangement, as the voltage increases, transistor Q8will become less conductive thus reducing the light output ofphoto-resistor lamp PRL. In turn, this will increase the resistance ofphoto-resistor PR in direct proportion to the increase in the supplyline voltage. The detector current is maintained at a constant valueover the entire range of supply line voltage from 102v AC to 132v AC.

Relay operating coil K1 controls single-pole double-throw switches SW-2and SW-3. A conductor 24, connected to isolated line conductor L1, isconnected, through a switch SW-l to a conductor 26 connected to switchSW-2. A conductor 25,

connected to isolated line conductor L2 is connected to one terminal ofa buzzer B2, and the other terminal of buzzer B2 is connected by aconductor 27 to switch SW-2. An "alarm ready neon lamp is connectedacross conductors 26 and 27. In the illustrated position of switch SW-Z,where relay energizing coil K1 is de-energized, neon lamp AR lights whena potential is applied to isolated line conductors L1 and L2. Uponoccurrence of a fault impedance, an energizing current is supplied torelay operating coil [(1, in the same manner as described with respectto FIG. 1, and switches SW-2 and SW-3 are transferred. The transfer ofswitch SW-2 energizes buzzer B2 to sound an alarm, and extinguishes lampAR.

Conductors 29 and 30 are connected to the opposite terminals ofsecondary winding S2 of transformer T1, with conductor 29 beingconnected to switch SW-3 and conductor 30 being connected to oneterminal of safe lamp SL and to one terminal of hazard" lamp HL.Respective conductors 31 and 32 connect the other terminals of these twolamps through switch SW-3. In the illustrated de-energized position ofrelay operating coil or winding Kl, switch SW-3 connects conductor 31 roconductor 29, so that safe" lamp SL is lit, and disconnects conductor 32from conductor 29 so that hazard lamp HL is not lit. Upon transfer ofswitch-SW-3 upon energization of relay coil or winding K1, conductor 32is connected to conductor 29 and conductor 31 is disconnected fromconductor 29. Consequently, hazard" lamp HL is lit and safe lamp SL isextinguished.

In this embodiment of the invention, a third secondary winding S5 isprovided on transformer T1 and provides a simple bias to bring meter MAback to a zero fault current reading, which is its own hazard index andwhich, with the illustrated circuitry, is 500 microamperes. This is fora 2 milliampere total hazard index, so that the detector hazard index isonly one-quarter or less of the total hazard index.

The various other connections shown in FIG. 2 are provided for testpurposes, including a test switch TS for connecting a test circuit TCbetween the isolated line conductors and ground to test the apparatus.Also, switch SW-l acts as a test switch for testing buzzer BZ. Otherterminals are provided for testing the various components of thecircuit.

Despite its very low detector hazard index and its high switching rateof 37 cps, the fault detector circuit of the invention is much simpler,more compact, and lighter in weight than known fault detector circuitspresently used as hazard indicators in operating rooms of hospitals andthe like.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:

1. For use with an AC supply line, having its line conductors isolatedfrom ground and subject to probable connection to ground by'anyconfiguration of resistive and capacitive fault impedances, a faultdetector circuit arranged for connection to said supply line and equallysensitive to balanced and unbalanced such fault configurations, saidfault detector circuit comprising, in combination, a first capacitor anda first resistor connected in series at a first junction point; a secondlike capacitor and a second like resistor connected in series at asecond junction point; said first capacitor and said second resistorbeing connected to one line conductor and said second capacitor and saidfirst resistor being connected to the other line conductor, and eachresistor having an impedance substantially equal in magnitude to that ofone of said capacitors at the supply line frequency; a third junctionpoint; switching and rectifying means operable to effect current flowfrom said first and second junction points alternately and cyclically tosaid third junction point; a source of DC potential; a variableimpedance connected in series between said third junction point and onetenninal of said source, and having an impedance varying directly withthe supply line voltage; a milliammeter connected between the otherterminal of said source and ground for alternating connection to saidfirst and second junction points to provide a path for current flowalternately from said line conductors in only one direction to groundthrough said milliammeter; whereby, upon occurrence of any configurationof resistive and capacitive fault impedances, said capacitors andresistors connected between said isolated line conductors will effect aflow of fault current through said milliammeter; and means operativelyconnected to said milliammeter and blocking current flow through saidmilliammeter in the reverse direction.

2. A fault detector circuit, as claimed in claim 1, in which at leastone of said first and second resistors is adjustable to compensate forphase shift.

3. A fault detector circuit, as claimed in claim 1, in which said sourceof DC potential comprises a full wave rectifier bridge having a pair ofAC input terminals connected to a source of AC potential; said oneterminal and said other terminal of said source comprising the otherterminals of said rectifier bridge.

4. A fault detector circuit, as claimed in claim 3, in which said sourceof AC potential comprises a transformer having a primary windingconnected across said isolated line conductors and a secondary windingconnected to said AC input terminals of said rectifier bridge.

5. A fault detector circuit, as claimed in claim 1, in which saidvariable impedance comprises a third resistor and a third capacitorconnected, in parallel with each other, between said third junctionpoint and said one terminal of said DC source.

6. A fault detector circuit, as claimed in claim 1, in which saidvariable impedance comprises a photo-resistor connected between saidthird junction point and said one terminal of said source, a lampoperable to direct illumination upon said photo-resistor, and meansconnected to said AC supply line and controlling the illumination ofsaid lamp as an inverse function of the supply line voltage, whereby theresistance of said photo-resistor varies as a direct function of thesupply line voltage to maintain the detector current constant.

7. A fault detector circuit, as claimed in claim 1, in which saidswitching and rectifying means comprises respective full wave rectifierbridges each having a first terminal connected to a respective one ofsaid first and second junction points and a second terminal connected tosaid third junction point;

respective transistors each having its output circuit connected to theother two points of a respective rectifier bridge; and an oscillatoreffecting alternate triggering of said transistors to the conductivestate.

8. A fault detector circuit, as claimed in claim 7, in which saidoscillator is a transistorized oscillator energized from said AC supplyline.

9. A fault detector circuit, as claimed in claim 3, including a secondfull wave rectifier bridge having a pair of opposite input terminalsconnected to said one terminal and said other terminal, respectively, ofsaid first-mentioned rectifier bridge; said milliammeter being connectedacross the other two terminals of said second full wave rectifierbridge; and means connecting one input terminal of said second full waverectifier bridge to ground.

10. A fault detector circuit, as claimed in claim 9, including a diodeconnected between said one and said other terminal of saidfirst-mentioned full wave rectifier bridge to bypass reverse currentflow therearound upon occurrence of a capacitive fault.

11. A fault detector circuit, as claimed in claim 1, including visualand audible hazard indicating means; relay contacts controllingconnection of said indicator means to a source of potential; a relayoperating coil controlling said relay contacts; and a transistoramplifier having an input connected to said milliammeter and an outputcircuit controlling connection of said relay operating coil to a sourceof potential; said transistor amplifier, responsive to a flow of faultcurrent through said milliammeter, operating said relay operating coilto move said relay contacts to a position connecting said hazardindicator means to a source of potential to energize the 1 2 A faultdetector circuit, as claimed in claim 11, in which said visual hazardindicator means comprises a hazard lamp; said audible hazard indicatormeans comprising a buzzer; and a safe" lamp controlled by said relaycontacts; said relay contacts, when said relay operating coil isdeenergized, connect ing said safe" lamp to a source of potential and,upon energization of said relay operating coil, disconnecting said safe"indicator lamp from the source of potential and connecting said hazardlamp and said buzzer to a source of potential.

1. For use with an AC supply line, having its line conductors isolatedfrom ground and subject to probable connection to ground by anyconfiguration of resistive and capacitive fault impedances, a faultdetector circuit arranged for connection to said supply line anD equallysensitive to balanced and unbalanced such fault configurations, saidfault detector circuit comprising, in combination, a first capacitor anda first resistor connected in series at a first junction point; a secondlike capacitor and a second like resistor connected in series at asecond junction point; said first capacitor and said second resistorbeing connected to one line conductor and said second capacitor and saidfirst resistor being connected to the other line conductor, and eachresistor having an impedance substantially equal in magnitude to that ofone of said capacitors at the supply line frequency; a third junctionpoint; switching and rectifying means operable to effect current flowfrom said first and second junction points alternately and cyclically tosaid third junction point; a source of DC potential; a variableimpedance connected in series between said third junction point and oneterminal of said source, and having an impedance varying directly withthe supply line voltage; a milliammeter connected between the otherterminal of said source and ground for alternating connection to saidfirst and second junction points to provide a path for current flowalternately from said line conductors in only one direction to groundthrough said milliammeter; whereby, upon occurrence of any configurationof resistive and capacitive fault impedances, said capacitors andresistors connected between said isolated line conductors will effect aflow of fault current through said milliammeter; and means operativelyconnected to said milliammeter and blocking current flow through saidmilliammeter in the reverse direction.
 2. A fault detector circuit, asclaimed in claim 1, in which at least one of said first and secondresistors is adjustable to compensate for phase shift.
 3. A faultdetector circuit, as claimed in claim 1, in which said source of DCpotential comprises a full wave rectifier bridge having a pair of ACinput terminals connected to a source of AC potential; said one terminaland said other terminal of said source comprising the other terminals ofsaid rectifier bridge.
 4. A fault detector circuit, as claimed in claim3, in which said source of AC potential comprises a transformer having aprimary winding connected across said isolated line conductors and asecondary winding connected to said AC input terminals of said rectifierbridge.
 5. A fault detector circuit, as claimed in claim 1, in whichsaid variable impedance comprises a third resistor and a third capacitorconnected, in parallel with each other, between said third junctionpoint and said one terminal of said DC source.
 6. A fault detectorcircuit, as claimed in claim 1, in which said variable impedancecomprises a photo-resistor connected between said third junction pointand said one terminal of said source, a lamp operable to directillumination upon said photo-resistor, and means connected to said ACsupply line and controlling the illumination of said lamp as an inversefunction of the supply line voltage, whereby the resistance of saidphoto-resistor varies as a direct function of the supply line voltage tomaintain the detector current constant.
 7. A fault detector circuit, asclaimed in claim 1, in which said switching and rectifying meanscomprises respective full wave rectifier bridges each having a firstterminal connected to a respective one of said first and second junctionpoints and a second terminal connected to said third junction point;respective transistors each having its output circuit connected to theother two points of a respective rectifier bridge; and an oscillatoreffecting alternate triggering of said transistors to the conductivestate.
 8. A fault detector circuit, as claimed in claim 7, in which saidoscillator is a transistorized oscillator energized from said AC supplyline.
 9. A fault detector circuit, as claimed in claim 3, including asecond full wave rectifier bridge having a pair of opposite inputterminals cOnnected to said one terminal and said other terminal,respectively, of said first-mentioned rectifier bridge; saidmilliammeter being connected across the other two terminals of saidsecond full wave rectifier bridge; and means connecting one inputterminal of said second full wave rectifier bridge to ground.
 10. Afault detector circuit, as claimed in claim 9, including a diodeconnected between said one and said other terminal of saidfirst-mentioned full wave rectifier bridge to bypass reverse currentflow therearound upon occurrence of a capacitive fault.
 11. A faultdetector circuit, as claimed in claim 1, including visual and audiblehazard indicating means; relay contacts controlling connection of saidindicator means to a source of potential; a relay operating coilcontrolling said relay contacts; and a transistor amplifier having aninput connected to said milliammeter and an output circuit controllingconnection of said relay operating coil to a source of potential; saidtransistor amplifier, responsive to a flow of fault current through saidmilliammeter, operating said relay operating coil to move said relaycontacts to a position connecting said hazard indicator means to asource of potential to energize the same.
 12. A fault detector circuit,as claimed in claim 11, in which said visual hazard indicator meanscomprises a hazard lamp; said audible hazard indicator means comprisinga buzzer; and a ''''safe'''' lamp controlled by said relay contacts;said relay contacts, when said relay operating coil is deenergized,connecting said ''''safe'''' lamp to a source of potential and, uponenergization of said relay operating coil, disconnecting said''''safe'''' indicator lamp from the source of potential and connectingsaid hazard lamp and said buzzer to a source of potential.